In conjunction with many electrical converter devices, e.g. a frequency converter, an inductive reactor arrangement is needed between an inverter bridge and an electrical system, e.g. an electrical motor, connected to the inverter bridge, and/or between a rectifier bridge and an alternating voltage network. The reactor arrangement can be needed, for example, for reducing slew rate of output voltages of an inverter, for over-current protection, for reducing radio frequency (RF) emissions, for reducing common-mode electrical currents, for suppressing disturbance electrical currents, and/or for suppressing harmonics of voltages and/or of electrical currents.
From the viewpoint of suppressing e.g. conductive RF-interferences and stray electrical currents it is, in many applications, important that a reactor arrangement provides sufficient common-mode inductance. The stray electrical currents may cause e.g. harmful electrical currents via bearings of rotating electrical machines supplied with a frequency converter. A conventional three-phase reactor arrangement according to the prior art comprises a three-leg magnetic core element made of magnetically amplifying material, i.e. material having the relative permeability greater than unity (μr>1). Windings for different phases are located around respective legs of the three-leg magnetic core element. The common-mode inductance of a reactor arrangement of the kind described above is relatively low, i.e. the ratio between the common-mode inductance and the differential mode inductance is low, because the magnetic core element does not provide a closed magnetic flux path for a common-mode magnetic flux component. Therefore, the common-mode magnetic flux component has to flow via air and/or other structures of the reactor arrangement. It is possible to provide the magnetic core element with one or more additional leg in order to provide a closed magnetic flux path for the common-mode magnetic flux component via the magnetic core element. Another conventional three-phase reactor arrangement according to the prior art comprises a five-leg magnetic core element. This kind of solution, however, increases the size and thus the weight of the magnetic core element.